There are mobile communications systems which include base stations (BS) and mobile stations (MS). The base stations provide wireless communication areas. In such mobile communications systems, the mobile stations have limited battery capacities. Accordingly, a general mobile communications system cancels the registration of a mobile station with the network if the mobile station does not perform communication for a predetermined period of time and suspends or restricts some functions of the mobile station. Here, the cancellation of the registration with the network includes the deletion of information regarding the mobile station from the base station by, for example, removing the connection of the mobile station to the base station and releasing assigned identification information of the mobile station. The registration with the network includes processes opposite to those performed for the cancellation of the registration. The identification information of the mobile station includes, for example, an IP address. According to the above-mentioned processes, the power consumption of the mobile station is reduced. This mode in which the mobile station operates is called an idle mode.
A mobile communications system to which WiMAX (Worldwide Interoperability for Microwave Access) is applied is described next as an example.
A mobile station and a base station which performs communication with the mobile station (i.e. preferred BS, hereinafter referred to as “source base station”) preliminarily determine a listening interval (communication interval) and an unavailable interval (non-communication interval). The mobile station in idle mode receives, during the listening interval, a paging advertisement message which is transmitted from the source base station and indicates the presence or absence of data directed to the mobile station. At this point, if reported by the paging advertisement that there are data directed to the mobile station, the mobile station wakes up from idle mode and performs a communication reestablishment process with the source base station in order to receive the data directed to the mobile station. The paging advertisement may be referred to as MOB_PAG-ADV. The source base station constitutes a paging group. The paging group includes multiple base stations. The following describes a case where a mobile station in idle mode moves and initiates communication with a base station belonging to a paging group different from a paging group to which a source base station of the mobile station belongs. In this case, the mobile station performs communication with a base station covering an area to which the mobile station has moved to (i.e. target BS, hereinafter referred to as “destination base station”). In the communication, the mobile station updates its location information (location update).
In order to communicate with a base station, the mobile station uses communication parameter information unique to the base station. The communication parameter information includes, for example, information regarding sub-channels used for data transmission and reception and a modulation scheme used for converting data into a signal. The communication parameter information includes a downlink channel descriptor (DCD) and an uplink channel descriptor (UCD). For example, the DCD includes a MAC message which specifies characteristics of the downlink physical layer, and the UCD includes a MAC message which specifies characteristics of the uplink physical layer.
In WiMAX, a base station distributes its DCD/UCD information, and also distributes DCD/UCD information of a base station adjacent to the base station as neighboring base station information periodically. The period is 30 seconds, for example. The neighboring base station information may be referred to as MOB_NBR-ADV. The mobile station obtains in advance the DCD/UCD information of the neighboring base station from MOB_NBR-ADV. Accordingly, when switching the connection destination to a destination base station (target BS), the mobile station is able to immediately initiate communication.    [Non-patent Document 1] IEEE802.16e
The following explains a case where the mobile station moves to a communication area of a destination base station before the neighboring base station information is distributed. Depending on the distribution timing of the neighboring base station information, the mobile station may switch the connection destination to the destination base station without receiving MOB_NBR-ADV. In this case, if the paging group to which the destination base station belongs is different from the paging group to which the source base station of the mobile station belongs, the mobile station does not have DCD/UCD information of the destination base station. Therefore, the mobile station waits until the DCD/UCD information is distributed by the destination base station. After receiving the DCD/UCD information from the destination base station, the mobile station updates its location information (location update). Therefore, if the mobile station moves to the communication area of the destination base station before the neighboring base station information is distributed, it may take some time for the mobile station to be able to initiate communication.